I've seen some threads discussing how 3.5 was problematic because save DCs are too high versus bad saves. I've seen other threads discussing how 3.5 was problematic because save DCs are too low for any of your lower level spells once you become higher level. Are either of these right? or is the problem something else?
My opinion: Having played spellcasters through 20th, I did find that I often couldn't use any spell with a save that negates unless it was one of the highest couple spell levels I could cast. I understand that people say you use those lower level spells against lower level encounters, but I don't often find myself facing goblins when I'm 15th. So, I'd spend all my time trying to boost my saves and do stat damage to other people's save stats.
On the other hand, for years, I've continuously heard people tell me wizards are overpowered, and by default, there shouldn't be any attempt to increase spell DCs of any kind.
I do get that spellcasters have an incentive to boost save DCs and that since they can increase only one stat (their spellcasting stat) and have an effect on save rolls against three different stats of the target, it is generally easier for spellcasters to boost than for targets to boost saves. Nevertheless, I don't know whether my DM arbitrarily boosted monster saves, but I never seemed to outstrip save DCs once we switched to 3.5 (which removed those class and feat powers that allowed easy boosting of save DCs).
I think the feel of overpowering might come not from save DCs, but in part from a wizard/cleric/druid's ability to pick powers suitable to a given situation on a daily basis, which other classes cannot do. Thus, they've always got something precisely useful in a certain situation.
I also think that save or die spells have a lot to do with the problem, since even a 40% chance success rate (the basic rate at which a 20th level target using its bad save saves against a 20th level caster using a 9th level spell), feels overpowering when failure is death. On the other hand, if you're a spell caster, it feels pretty cheap not to have at least a 50/50 chance at your best spell working on an equal opponent. Especially when you feel like you can't use the bottom 2/3rds of your spells.
Also, some of this feeling may come from the fact that, other than direct damage, saves usually negate, which makes magic use very swingy. Whether it's death or blindness, having either a wasted action or a serious condition be the possible options is going to be swingy by necessity.